This has consequences for proponents of the local food movement and for those who simply wish to reduce their carbon footprint: buying local will certainly help, but not nearly as much as choosing foods made by low-carbon production methods. This means that if you really want to lower your carbon footprint, you should eat more vegetables and fewer animal products, particularly red meat and cow’s milk.

But transportation still does clearly matter: even 11% of food industry greenhouse gas emissions is a lot of greenhouse gas.

So, this raises our next question: What’s the best way to reduce greenhouse gas emissions in food transportation?

The question is not as simple as it first seems. There’s a lot of talk about “food miles,” or how many miles your food, and its ingredients, travel before they get to your plate. But not all “food miles” are equal. Air transport emits hundreds of times the greenhouse gas per mile that rail transport emits. Trucking emits over eight times more greenhouse gas than shipping across lakes.

Fortunately, air transport for food is rare: Michael Pollan’s notorious Argentine asparagus may, due to its perishability, have been flown to his local market. Kiwis from New Zealand may receive the same star treatment. But this is by far the exception, and accounts for well under 1% of all “food miles” expended getting food to our plates in the U.S.

But there are notable differences even outside of air shipment. Trucking makes up only 29% of all food miles but generates 71% of our food-transport greenhouse gas emissions. This is extremely costly. Even shipping by rail, which is twelve times greener than trucking, emits 50% more greenhouse gases than shipping by international waters.

Why do we care?

Well, this should inform our debate on how best to cut food-transport greenhouse gas emissions. International shipping has been greatly on the rise in recent decades, and has caused a big stir. But shipping products from one continent to another may emit way less greenhouse gas than simply trucking food from one U.S. state to another. And shipping from Europe to New York by sea can emit less greenhouse gas than shipping by rail from California to New York.

So, what do we do?

There is certainly room to improve our footprint by buying locally. If you live near a farm, you’ll have a lower footprint by buying locally than from buying from overseas markets.

But that room to improve, without major changes in our diet, is somewhat limited. Not everyone lives near farms, and many climates are not amenable to year-round agricultural production. It is environmentally costly to ship from a farm in upstate New York to New York City consumers; New York City consumers can receive water-transported food from 18 times farther away at the same cost to the environment. And it may be too much to ask people in Canada who may farm only a few months a year, to freeze their food and eat it throughout the winter, rather than receive fresh produce from Arizona during the colder months.

It's Easy To Buy Local in Santa Monica. It's Harder in Other Places.

So, we can certainly take incremental steps by eating locally. But we may need far bigger changes in diet patterns before we can make a large dent in this 11% of food industry greenhouse gas emissions.